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Diana Falls slip recovery

7 September 2018Kate Palmer

Responding quickly to damage caused by storms is something you need to be good at on the West Coast and there is an increasing requirement to build in resilience following storm events, which are set to become more frequent and extreme with climate change.

Not only is WSP Opus an international leader in designing infrastructure that’s resilient to the impacts of climate change, on the ground they have the local expertise to deliver innovative solutions and are prepared to get their hands dirty on repair efforts.

That was certainly the case when torrential rain caused a major slip at Diana Falls in the Haast Pass in 2013, noted as the most complex and hazardous slip in New Zealand. The slip left over 40,000 cubic metres of rock and debris on the highway, and closed State Highway 6 for 11 days, causing significant social and economic impact to local communities. The NZ Transport Agency engaged WSP Opus to help get the road open as soon as possible, with a workable solution in place in a reasonable timeframe.

There were challenges in abundance. It involved potentially hazardous activities just to keep the road open before remedial works could be started. Then there was the extreme climate of the winter months, complex geology, the remoteness of the site and the ever-present risk of further slips.

The recovery was a massive effort in which WSP Opus played a key role, quickly identifying that if nothing was done, further large falls of rock and soil would continue to affect the highway. Daily helicopter inspections (weather permitting) were carried out by geotechnical engineers to monitor the upper sections of the landslide where significant tension cracks had been observed. It was also recognised that the potential risk to staff on site and the travelling public due to the slip hazard was high, so an effective process was developed to ensure site safety.

Building a rock fence was decided as the most feasible long term option as it manages the falling material, slowing it and allowing to come down the slope in a controlled and safe fashion.

This highly innovative engineering solution turned out to be the most complex rock fall protection system ever seen in the Southern Hemisphere and was designed and delivered by WSP Opus.

Each of the three rock fall protection fences built on the slip face is capable of stopping a boulder weighing up to 16 tonnes (the size of a small car) travelling down the hillside at a speed in excess of 90km/h.

The success of the project delivery was a result of the entire project team, including NZ Transport Agency, WSP Opus and Sicon Fergusson working together to allow the road to permanently reopen to the public.